I'll speak towards teas and teas only, as grinds affect extraction the most; toss and wash effects are entirely different than tea with the plant matter. Different grinds and many other variables (body pH, stomach contents, metabolism, potentiators, etc.) play a role in toss and wash. The body has a much more complex extraction process; I'm not speaking to this. The goal is to help someone determine what grind is best for them in making tea for the best extraction possible based on the grind, especially if using an aeropress.
But first, we need to walk through some context. I'm making some assumptions that the tea is just below a boil (at a simmer) at 170-180 degrees. The temperature of 170-180 is used to break down a lot of the plant matter without allowing the heavier alkaloids to evaporate with the vapor in a violent boil. I'm assuming this analysis is for same weight for fine, coarse, and crushed leaf (they're all sold similar price for weight).
Extraction depends completely on two things, time and surface area.
Time - Though kratom is not as water soluble as most things, the longer it is in water, the more alkaloids are extracted. The temperature helps release these from the plant matter, but it needs time as well. Obviously, there's a point at which more time equals less benefit, but the idea is to ensure the powder is exposed for as long as possible (usually what you feel comfortable waiting on).
Surface area - increasing surface area also makes it easier to absorb alkaloids. When one has a finer grind, you are increasing the surface area exponentially. Don't believe me? Just look at espresso vs drip coffee, I'll continue this analogy later.
For example, when eating a watermelon, one cuts it in half, halves that, halves that again, each time increasing the surface area that you can take a bite out of. Math is hard in public, so I'll use a Japanese square watermelon. A 10in square water melon has a surface area of 600in2 (10inx10in on 6 sides). Cutting it in half once, cutting it again (leaving quarters), and third time (eighths) leaves you with (8)5in cubes, each having a surface area of 150in2 (5inx5in on 6 sides) for a total surface area of 1200in2. Just reducing a particle to an 1/8th of its size doubles the surface area. A way to compare is table salt is usually about 100 micrometers, where talc powder is 10 micrometer (a tenth and even more surface area than comparison above).
Grind size doesn't affect what is being extracted, it changes how fast the extraction occurs. Think of a coffee bean, we don't use them unground, a coffee brew would take a very long time; we grind them to make the extraction faster. We grind them further to make espresso (even faster than drip).
A finer grind will allow faster absorption. A coarser grind will allow slower absorption. When you want to increase extraction, you need to grind finer. You need to increase the surface area to "show" the water more of the alkaloids. With surface area, extraction will increase because the water can ‘see’ more of the alkaloids and get to work absorbing them. It's not so dissimilar absorption from the body, but I'll hopefully get to that one in a later post.
With coarse grinds, it's harder for the water to work itself inside to reach all the parts to dissolve the alkaloids. The heat will break down some of the plant matter, but not to the extent of grinding finely, and it will be exponentially faster with finer grinds (as the water can now "reach" more parts to break down).
It's science, we do this for other tea, matcha, coffee, sugar.... etc... Finer sugar absorbs faster into water, coarse is slower.
So on to the fun part. What does this mean for the aeropress? If using the aeropress with powder on the filter and not simmering powder in the water before, one would need to use finer grinds to extract quicker as the hot water only has a few seconds to be pushed out the other side. This is enough to extract. If finer grinds are unavailable, one needs more time (brewing tea longer on the stove first).
What does this mean for tea? If you're pressed for time, finer grinds are superior in extraction over short duration to the coarse grinds in the tea pot. If you don't mind the 30-40 min wait for a regular tea routine, by all means coarse is for you.
Other observations for teas: Crushed leaf is way easier to strain than powder, but has far less surface area, you need to brew for a long, long time to get the most bang for your buck. If you can strain a powder, I'd recommend doing so.
With the same weighed amount of each, over a shorter time, you'd end up with more alkaloids in a cup of tea with finer grind due to extraction (with the same variables of heat and process)
Short, I love my aeropress and it works well with the finest grinds I can find and quickly, but not much else (otherwise it's a fancy strainer). For those that haven't seen the watermelons:
